HTML
HTML, the predominant markup language for web pages, has no mechanism for marking up notes. Despite a number of different proposals over the years, and repeated pleas from the user base, the working group has been unable to reach a consensus on it. Because of this, MediaWiki, for example, has had to introduce its own tag for citing references in notes, an idea which has since also been implemented for generic use by the Nelson HTML preprocessor.
It might be argued that the hyperlink partially eliminates the need for notes, being the web's way to refer to another document. However, it does not allow citing to offline sources and if the destination of the link changes, the link can become dead or irrelevant.
Read more about this topic: Note (typography)